This invention relates to an improved process for molding articles. More specifically, this invention relates to an improved mold-coating step wherein a one component, room temperature curing silicone release composition is applied to the mold surfaces to provide multiple release of articles therefrom.
During a molding process, the solid article produced often sticks to the shape-determining surfaces of the mold thereby interfering with its separation from the mold. This sticky problem is particularly prevalent in molding processes which use liquid, organic molding compositions which are curable to the solid state, such as polyurethanes, unsaturated polyesters and polyepoxides. The liquid state of the molding composition allows it to achieve intimate contact with, and even wet, the shape-determining surfaces of the mold, thereby favoring adhesion of the molded article to said surfaces by a physical process. The curing process of the molding composition is an aggressive chemical process which has the potential of involving the shape-determining surfaces of the mold to some extent, thereby further favoring adhesion of the molded article to said surfaces by a chemical process. For these reasons the molding of articles from curable liquid compositions presents release problems which are much more severe than those encountered during the molding of non-curing compositions, such as thermoplastic materials. Furthermore, in a closed-mold process, wherein the confined curing liquid composition may be subjected to pressure, such as in the preparation of high-resiliency polyurethane foam articles, such as automobile seat cushions, furniture upholstery and impact pads, said adhesion-favoring processes are accentuated.
For over twenty years silicone compositions have been used as mold release agents. However, several problems have been associated with silicones which have restricted their commercial utility especially in the area of multiple release for high resiliency polyurethane foam articles. Uncured silicone compositions, such as polysiloxane waxes, greases and fluids, transfer to the molded article during demolding and need to be frequently reapplied to the mold. Such compositions containing uncured organopolysiloxane fluid also have a tendency to defoam polyurethane foam compositions at the mold surface and produce an undesirable skin on the molded article.
Cured silicone compositions, while not being transferable, have lacked sufficient abrasion-resistance, either because they are not tough enough or because they do not adhere sufficiently to the mold, so that they also need to be frequently reapplied. Potentially more permanent silicone compositions have failed because components in the molding composition act as inhibitors to prevent the cure of the silicone release composition when it is applied to mold surfaces. In still other cases, catalysts in the molding composition have interacted with the silicone release composition to rapidly alter the silicone surface so that the surface characteristics of successively molded articles rapidly deteriorate.
Braley, U.S. Pat. No. 2,811,408, teaches that a resin of trimethylsiloxane units and SiO.sub.2 units, optionally containing an organopolysiloxane, provides excellent release of articles molded from organic and organosilicon plastics when applied to the surface of the mold. No provision is made for curing the optional organopolysiloxane when used, and transfer from the mold surface to the molded article will occur.
Oppliger, Canadian Pat. No. 624,114 teaches that cured polydimethylsiloxane coatings can be employed as a release agent on molds for conventional polyurethane foam articles. Specifically, Oppliger describes compositions in which polydimethylsiloxane fluids are cured by crosslinking agents such as methylhydrogensiloxanes, alkylsilicates and alkylpolysilicates using as catalysts organotin salts of carboxylic acids. These compositions can be commercialized only as two part systems because of the limited pot life after mixing the components.
Brooks, U.S. Pat. No. 3,684,756 discloses a resinous mold release composition composed of methyl-terminated polydimethylsiloxane, a partially methoxylated methyl and propyl polysiloxane resin, and a curing agent for the resin such as tetraalkyl titanates or organotin salts of carboxylic acids. While this composition is said to give multiple releases of plastic articles from a mold treated therewith, the methyl-terminated polydimethylsiloxane possesses no curing sites and is not cured in or cocured with the resin. Transfer of the uncured methyl-terminated polydimethylsiloxane from the mold surface to the surface of the molded article is likely, thus leading to the problems of transfer and defoaming discussed above.
Battice, U.S. Pat. No. 4,244,912 teaches a mold release composition composed of vinyl-endblocked polydimethylsiloxane, a xylene-soluble vinyl-containing resinous polysiloxane, a methylhydrogenpolysiloxane crosslinking agent, and a platinum hydrosilylation catalyst. The curing of the polydimethylsiloxane into the resin system avoids the problems of transfer and defoaming, but the platinum catalyst of this composition is easily inhibited by amines and other components of the molding compositions so that reliable cures have not always been obtainable in some molding operations.
Heine, U.S. Pat. No. 3,492,394 teaches the application of organic solvent solutions of perfluoroalkyltrialkoxysilane to mold surfaces to provide durable release coatings. It is suggested that the hydrolyzable groups react with adventitious water on the mold surfaces to form cured films.
Ceyzeriat et al., U.S. Pat. No. 3,151,099 teach silicone compositions that are stable in the absence of moisture but cure spontaneously upon exposure to moisture. The compositions are prepared from hydroxyl terminated polydimethylsiloxane fluids, a silicic ester such as ethylorthosilicate, and a titanate or zirconate ester such as tetrabutyltitanate. Ceyzeriat et al. do not suggest that the compositions can be used as mold release coatings, in contrast they state that the compositions are suitable especially for sticking various articles such as Bakelite, metals, masonry, wood and glass.
Brown et al., U.S. Pat. No. 3,161,614 also teaches silicone compositions that are stable in the absence of moisture but cure spontaneously upon exposure to moisture. The compositions are composed of polydimethylsiloxane terminated by a siloxane unit having at least two alkoxy substituents and a curing catalyst such as titanium esters.
Weyenberg, U.S. Pat. No. 3,334,067 teaches a one part silicone composition that is stable in the absence of moisture but spontaneously cures in the presence of moisture. The compositions are prepared by mixing in the absence of moisture, a hydroxy terminated polydimethylsiloxane, methyltrimethoxysilane, and a beta-dicarbonyl titanium chelate compound. Weyenberg teaches that the use of other titanium compounds results in substantial increases in viscosity when hydroxylated siloxane is first mixed with the titanium compound such that stable mixtures can not be reliably prepared even in the absence of moisture.
It is an object of this invention to provide an improved process for molding articles which permits the release of multiple articles between successive applications of a release coating to the mold. It is another object of this invention to provide a one part mold release composition that is stable in the absence of moisture but spontaneously cures when applied to the mold surface and exposed to atmospheric moisture. It is also an object of this invention to provide a mold release coating that does not transfer from the mold surface to the molded article. It is also an object of this invention to provide a mold release coating which will not defoam polyurethane foam during the molding process.